Ophthalmic examinations are commonly made for the early diagnosis of various diseases that come before lifestyle-related diseases and causes of blindness. A tomography apparatus for an eye portion such as an OCT (Optical Coherence Tomography) is expected to be effective to give more adequate diagnoses of diseases since it allows to three-dimensionally observe the state of the interior of retina layers.
A CAD (Computer-Aided-Diagnosis) technique using a computer is needed for the purpose of reducing the loads on diagnoses by doctors using the OCT. Functions required for the CAD for the OCT include, for example, a function of acquiring the three-dimensional (3D) position, shape, and distribution state of a morbid portion from an OCT image so as to grasp the degree of progress of a disease. Another example of the functions includes a function of presenting a temporal change associated with, for example, the shape of a morbid portion in correspondence with a set of OCT images of a single eye to be examined, which are captured at different times.
An image processing unit which implements such advanced diagnosis assistant functions is generally premised on that a morbid portion is extracted from an OCT image in pre-processing. Since the precision of analysis results to be presented is influenced according to this extraction precision of the morbid portion, implementation of high-precision morbid portion extraction processing has a very important implication. The present invention focuses attention on an exudate as one of lesions of eye portions.
A method to detect an exudate from a 3D OCT image with high precision has not been proposed yet. In general, an exudate has, as image features, a higher intensity than a surrounding portion and a massive structure. However, the OCT image includes locally high-intensity regions such as a nerve fiber layer, a junction between inner and outer photoreceptor segments, and a surrounding portion of a retinal pigment epithelium boundary in addition to the morbid portion. The current image quality of the OCT image does not have a sufficient contrast between the morbid portion and its surrounding portion. For this reason, it is difficult to extract an exudate with high precision by only focusing attention on the aforementioned image features.
As one solution for such problem, a region that is to undergo image analysis is limited in advance using information of the morbid portion obtained by another modality. A means for specifying a region to be limited in this way is disclosed in Japanese Patent Laid-Open No. 2007-325831.
FIG. 1 is a schematic view for explaining the invention disclosed in Japanese Patent Laid-Open No. 2007-325831, and shows a typical cross-sectional image (B-scan image) of an OCT image. FIG. 1 illustrates an inner limiting membrane 101, a boundary 102 between an optic nerve fiber layer (stratum opticum) and its underlying layer, a junction 103 between inner and outer photoreceptor segments, a retinal pigment epithelium boundary 104, and a blood vessel 105 in retina layers. In Japanese Patent Laid-Open No. 2007-325831, a blood vessel region is extracted from a fundus image of a single eye to be examined, so as to detect a blood vessel of an OCT image (not a morbid portion), and that position is projected onto the OCT image, thereby specifying a region where the blood vessel exists in the OCT image. As can be seen from FIG. 1, a region between broken lines 106 and 107 is a region obtained by projecting, onto the OCT image, the blood vessel region obtained from the fundus image, and the region where the blood vessel exists is specified.